Timing and mechanisms of surface and intermediate water circulation changes in the Nordic Seas over the last 10,000 cal years: a view from the North Iceland shelf

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Abstract

The North Iceland shelf bears essential components of the present surface and intermediate circulation of the northern North Atlantic. Instrumental and historical data give evidence of the sensitivity of this domain to broad, regional-scale oceanic and atmospheric anomalies. Our investigation of the paleohydrological variability off Northern Iceland throughout the last 10 000 cal yr suggests that atmospheric forcing alone, through combined changes in strength of the wind stress curl and sea-level atmospheric pressure pattern over the Nordic Seas, is sufficient to explain the recorded changes in origins and dynamics of surface and intermediate water masses. Our biotic proxies, coccoliths and benthic foraminifera, were extracted from a giant piston core (MD99-2269) collected in a shelf trough where sediment accumulated at an excess rate of 2 m/kyr. The mid-Holocene from 6.5 to 3.5 cal kyr BP was a time of peaked carbonate production and subsequent sedimentation, and strong water-column stratification with a thick layer of cold-fresh Arctic surface water overlapping an enhanced flow of Irminger/Atlantic Intermediate water. Applying conditions triggering present-time carbonate plankton blooms in the studied area, we infer that a lowered cyclonic activity associated with decreased winter storms and reduced production of Arctic Intermediate Water in the Iceland Sea were conductive of the recorded mid-Holocene water column structure. The opposite situation (warm Atlantic surface water, low vertically-integrated inflow of Irminger water, abutment of Arctic Intermediate water in deep shelf troughs) characterized the early Holocene as well as a shorter late Holocene period centred at 2 cal kyr BP. The Little Ice Age (ca. 0.2–0.6 cal kyr BP) and a short event at around 3 cal kyr BP stand as times of extreme advection of polar waters and extended sea–ice development. A comparison of the recorded long-term Holocene evolution of water column structure off Northern Iceland with climate and hydrological changes in the north-eastern Atlantic suggests that the strength of Atlantic inflow into the Nordic Seas was subjected to a balance between the Irminger and the Norwegian branches. This balance is thought to be mostly related to changes in the intensity and location of westerly winds and associated atmospheric pressure gradients in the North Atlantic.

Introduction

The North Iceland shelf has recently been the subject of intensive marine geological investigations which showed that this restricted area encapsulated, throughout the last deglaciation and Holocene periods, oceanographic and atmospheric variability that occurred over a much broader area (Andrews et al., 2000; Jennings et al., 2002; Andrews and Giraudeau, 2003; Andersen et al., 2004). Historical, as well as instrumental records have highlighted the sensitivity of this domain to recent oceanic and atmospheric anomalies such as the Great Salinity Anomaly (GSA) in the late 1960s, or the Little Ice Age (LIA) cold spell dated off northern Iceland at 750–100 cal yr BP (Knudsen and Eiriksson, 2002). Both events shared common features off northern Iceland including surface water cooling and freshening associated with increased influence of arctic and polar waters (Dickson et al., 1988; Olafsson, 1999; Eiriksson et al., 2000). Those surface water changes have a profound effect on both primary productivity and sea-ice extent as evidenced by hydrographic survey conducted in these waters over the last 50 yr (Thordardottir, 1984). In addition, it has been shown that surface water changes on the North Iceland shelf are tightly associated with altered overturning of deep and intermediate waters in the Iceland and Greenland seas (Malmberg and Jonsson, 1997), as well as with variations in the flux of Atlantic waters entering the Norwegian Sea (Blindheim et al., 2000). Atmospheric forcing, through variability in strength of the wind stress curl over the Nordic Seas (Jonsson, 1992), as well as changes in the state of the atmospheric pressure system over the North Atlantic (Dawson et al., 2003; Blindheim et al., 2000), are seen as the main drivers of changes in the water-mass structure of the northern North Atlantic.

The North Iceland shelf bears essential components of the present surface and deep circulation of the northern North Atlantic (Fig. 1; Hopkins, 1991; Stefansson, 1962). It is located close to the Arctic Front which separates Arctic/Polar water masses carried by the south-eastward flowing East Iceland Current (EIC) from the North Iceland Irminger Current (NIIC), a branch of the Irminger Current which rounds the western side of Iceland and feeds the North Iceland shelf with warm Atlantic waters. The NIIC reaches the inner part of the North Iceland shelf while the deeper realms are occupied by Arctic Intermediate waters (AIW), down to approximately 500 m, which is formed by convection in the Iceland and Greenland seas (Rytter et al., 2002; Malmberg and Jonsson, 1997). Starting from the mid-shelf area, Atlantic Water carried in the NIIC is submerged beneath colder, fresher Arctic Water carried by the EIC (Fig. 1). This water-column stratification is well displayed in Hunafloaall, where the wedge of Atlantic Water between 100 and 350–400 m water depth is cooled to 3.5 °C and freshened to 34.9‰. This submerged vestige of the NIIC is termed Irminger Intermediate Water (IIW). Beneath the IIW, temperatures decline with depth from 3.5 °C to slightly lower than 0 °C in the deepest parts of the northern shelf troughs. Salinities are 34.8‰, less than those in the IIW. The bottom water mass represents upper Arctic Intermediate Water (Swift, 1986), although the deepest parts of the northern troughs may contain Norwegian Sea Deep Water (Rytter et al., 2002).

We investigate the water-column dynamics off NW Iceland throughout the last 10 000 cal yr using the combined records of coccoliths, proxies for surface water conditions, and benthic foraminifera as tracer of intermediate and bottom water masses in the nearby Nordic Seas. Our higher time resolution proxy in the present study, coccoliths, has been successfully tested in recent works dealing with the Holocene short and long-term evolution of the North Atlantic Drift south of Iceland (Giraudeau et al., 2000), and of its NIIC branch over the inner North Iceland shelf (Andrews and Giraudeau, 2003). It complements another surface water proxy, diatoms, which have been recently investigated in MD99-2269 (Andersen et al., 2004). Recent works on the modern and fossil distribution of benthic foraminifera around Iceland highlighted the close correspondence of species distribution with bottom water-masses and their characteristics (Rytter et al., 2002; Jennings et al., 2002; Jennings et al., 2004).

Section snippets

Material, core chronology and methods

Giant piston core MD99-2269 (66° 37′N–20° 51′W) was collected as part of the IMAGES V cruise of RV Marion Dufresne. The core was retrieved at 365 m water depth from a 30 m thick sediment unit on the floor of Hunafloaall, a north–south orientated depression off N/NW Iceland. Previous works have shown that this core contains a continuous Holocene sedimentary series which accumulated at a rate close to 2 m/kyr (Andrews et al., 2003a). The construction of the shelf sediment unit and the recorded excess

Coccolith species distribution and coccolith carbonate

Coccolith species diversity is typically low as expected for this arctic/subarctic setting (Baumann et al., 2000; Andrews and Giraudeau, 2003). Dominance is equally shared between Coccolithus pelagicus, the cold-end member of the coccolithophore community in North Atlantic waters, and the ubiquitous Emiliania huxleyi (Fig. 2) which is presently responsible for extensive blooms in transitional/subarctic waters as well as in shallow settings along Norway and NE America (Brown and Yoder, 1994).

Discussion and conclusions

The combined coccolith and benthic foraminifera Holocene records are indicative of a clear antagonism in physical–chemical status and sources of water masses between the surface and bottom layers of the water column. In the following discussion, we interpret the Holocene evolution of water-column structure over the North Iceland shelf as a direct consequences of coupled changes in outflow of Arctic bottom and surface waters from the Nordic Seas and inflow of Atlantic waters around western

Acknowledgements

Core MD99-2269 was recovered in 1999 as part of the GINNA/IMAGES V, Leg 3 cruise of the RV Marion Dufresne. We gratefully acknowledge Yvon Balut (IPEV) and J.-L. Turon (DGO, chief scientist) for the outstanding coring operations. The cruise was supported by the French Polar Institute (IPEV) and USA participation was funded through grant NSF-OCE98-09001 and by NSF-OPP-0004233. This paper is also a contribution to an Earth System History (ESH) grant from NSF ATM-0317832. We thank D. Ostermann

References (56)

  • J. Giraudeau et al.

    Distribution and malformation of living coccolithophores in the northern Benguela upwelling system off Namibia

    Marine Micropaleontology

    (1993)
  • J. Giraudeau et al.

    Coccolith evidence for instabilities in surface circulation south of Iceland during Holocene times

    Earth and Planetary Science Letters

    (2000)
  • K. Grönvold et al.

    Ash layers from Iceland in the Greenland GRIP ice core correlated with oceanic and land sediments

    Earth and Planetary Science Letters

    (1995)
  • T.S. Hopkins

    The GIN Sea—a synthesis of its physical oceanography and literature review 1972–1985

    Earth Science Reviews

    (1991)
  • K.L. Knudsen et al.

    Application of tephrochronology to the timing and correlation of palaeoceanographic events recorded in Holocene and Late Glacial shelf sediments off North Iceland

    Marine Geology

    (2002)
  • H.H. Lamb

    Climatic variations and changes in the wind and ocean circulationThe Little Ice Age in the Northeast Atlantic

    Quaternary Research

    (1979)
  • J.E. Wollenburg et al.

    Living benthic foraminifers from the central Arctic Oceanfaunal composition, standing stock and diversity

    Marine Micropaleontology

    (1998)
  • J.R. Young et al.

    Calculation of coccolith volume and its use in calibration of carbonate flux estimates

    Deep-Sea Research II

    (2000)
  • Aagaard, K., 1972. On the drift of the Greenland pack ice. In: Karlsson, T. (Ed.), Sea Ice. Proceedings of an...
  • C. Andersen et al.

    Non uniform response of the major surface currents in the Nordic Seas to insolation forcingimplications for the Holocene climate variability

    Paleoceanography

    (2004)
  • J.T. Andrews et al.

    Distribution, sediment magnetism, and geochemistry of the Saksunarvatn (10.18±cal ka) tephra in marine, lake, and terrestrial sediments, NW Iceland

    Journal of Quaternary Science

    (2002)
  • J.T. Andrews et al.

    A high-resolution Holocene sediment record from Hunafloaall, N Iceland margincentury- to millennial-scale variability since the Vedde tephra

    The Holocene

    (2003)
  • H.A. Andruleit

    A filtration technique for quantitative studies of coccoliths

    Micropaleontology

    (1996)
  • Andruleit, H.A., 2000. Dissolution-affected coccolithophore fluxes in the central Greenland Sea (1994/1995). Deep-Sea...
  • G. Bond et al.

    Persistent solar influence on North Atlantic climate during the Holocene

    Science

    (2001)
  • K.R. Briffa et al.

    A 1400-year tree-ring record of summer temperatures in Fennoscandia

    Nature

    (1990)
  • C.W. Brown et al.

    Coccolithophorid blooms in the global ocean

    Journal of Geophysical Research

    (1994)
  • B.H. Corliss

    Microhabitats of benthic foraminifera within deep-sea sediments

    Nature

    (1985)
  • Cited by (0)

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